1. Field of the Invention
The present invention relates to an improvement in output power characteristics to various lights, of a power supply device powered by a solar cell and adapted for use in an electronic desk calculator or the like.
2. Description of the Prior Art
In a device powdered by a solar cell, for example a desk calculator, a solar cell 1 is usually connected, as shown in FIG. 1, to a processor 3 in said calculator through a voltage regulating circuit 2. The solar cell 1 is generally composed of so-called single crystal silicon solar cell elements prepared by forming PN junctions on a silicon substrate by impurity doping or amorphous solar cell elements prepared by evaporating amorphous silicon on a glass or metal substrate by glow discharge through monosilan, and such elements are serially connected in a number required for obtaining a voltage corresponding to the drive voltage of said processor 3.
The voltage regulating circuit 2 reduces the voltage V.sub.IN generated by the solar cell 1 to the driving voltage V.sub.OUT required for driving the processor 3 of the calculator and supplies a stabilized voltage to said processor. FIG. 2 shows the operational characteristic of the voltage regulating circuit 2, wherein a voltage margin for stable function of the processor 3 is defined by voltages V.sub.OUT1 and V.sub.OUT2. In case the voltage supplied from the solar cell 1 does not reach a value V.sub.IN1, the corresponding output voltage does not reach the lower limit voltage V.sub.OUT1 required for the processor 3, so that the calculator is unable to perform the normal function. FIG. 3 shows a characteristic curve A of a solar cell representing the relationship between the illuminance of illumination intensity L on the light-receiving face thereof and the generated voltage V.sub.IN.
FIG. 3 indicates that a minimum illuminance L1 is required for obtaining a voltage exceeding V.sub.IN1 shown in FIG. 2. Stated differently, the solar cell-powered calculator shown in FIG. 1 functions only under a light with a determined illuminance.
FIG. 4 shows the distribution of relative intensity to wavelength of the energy radiated by the sun, a white fluorescent lamp and a tungsten lamp, which are the most common light sources for the solar cell 1. The sunlight has a uniform coverage from the shorter ultraviolet wavelength region to the longer infrared wavelength region, including the visible wavelength region (450-680 nm) sensitive to the human eyes. On the other hand, the light from the white fluorescent lamp contains the components in the visible and ultraviolet wavelength region but not the components in the infrared wavelength region. Also the light from the tungsten lamp, or incandescent lamp, principally contains the components in the infrared wavelength region, and the components in the ultraviolet and visible wavelength regions are considerably less than those in the solar light or in the light from the white fluorescent lamp. Besides, the solar cells have different spectral sensitivities according to the species.
FIG. 5 shows examples of such spectral relative sensitivities, for the single-crystal silicon solar cell and the amorphous silicon solar cell, which are most popularly employed in practice. The single-crystal silicon has a high sensitivity to the light of the longer infrared wavelength region but is almost insensitive to the light of the shorter ultraviolet wavelength region. On the other hand the amorphous silicon is highly sensitive to the light of the visible and ultraviolet wavelength regions but is totally insensitive to the light of the infrared wavelength region. Consequently, as will be understood from FIGS. 4 and 5, the single-crystal silicon solar cell is capable of providing a sufficient power for driving the calculator with the light from a tungsten lamp, even under a low illuminance condition, since the spectral distribution of the light components matches the spectral sensitivity of the solar cell. However, in a situation utilizing the white fluorescent light as the light source, the generated power is reduced because of the increased component in the short wavelength region, so that the illuminance required for obtaining the voltage V.sub.IN1 for driving the calculator increases to L2, which, as shown by the curve B in FIG. 3, is higher than L1.
On the other hand, the amorphous silicon solar cell provides sufficient power even under low illuminance light from the white fluorescent lamp since the spectral distribution of the light components matches the spectral sensitivity of the solar cell, but requires a higher illuminance L3 in FIG. 3 for obtaining the aforementioned voltage V.sub.IN1 when powered by the tungsten lamp which provides light components mainly in the long wavelength region, thus resulting in a voltage-illuminance curve C shown in FIG. 3.
Consequently the conventional desk calculators powered by solar cells require proper selection of the light source as the illuminance required for the proper function depends significantly on the species of the light source, and, in fact, such calculators could not be used in certain locations.